Conventionally, such a electromagnetic flowmeter has an exiting coil, which forms a magnetic field in a direction perpendicular to a flow direction of fluid flowing inside a measuring pipe, and signal electrodes which are provided on an inner circumference face of the measuring pipe so as to face a direction perpendicular to the magnetic field formed by the exiting coil, wherein an electromotive force, which is generated in the fluid flowing through the inside of the measuring pipe due to the magnetic field formed by the exiting coil, is taken out from the signal electrode (for example, refer to Japanese Patent Application Publication No. 4-319622).
A main section of such an electromagnetic flowmeter of the prior art is shown in FIG. 7.
In the figure, a measuring pipe 10 comprises a nonmagnetic metal pipe 20 (for example, a pipe made of nonmagnetic stainless steel), and a lining 30 formed on an inside of the nonmagnetic metal pipe 20. In this example, insulating resin powder is coated on the inside of the nonmagnetic metal pipe 20 by powder coating, thereby forming the lining 30. The signal electrodes 40 and 40 are provided so as to face the inner circumference face of the measuring pipe 10.
In addition, as schematically shown by a dashed-dotted line in FIG. 7, an exiting coil CL, which forms a magnetic field in a direction perpendicular to a flow direction of fluid flowing inside the measuring pipe 10, is provided. The signal electrodes 40 and 40 are provided so as to face in a direction perpendicular to the magnetic field formed by the exiting coil CL.
First Example of Installation Structure of Signal Electrode
A first example of an installation structure of one of signal electrodes 40 attached to an inner circumference face of a measuring pipe 10 is shown in FIG. 8. In this first example, an electrode section 40a of the signal electrode 40 is in a plate shape, and, an axis portion 40b is provided so as to project from the center of a rear face of the plate shape electrode section 40a. The signal electrode 40 is attached to the inner circumference face of the measuring pipe 10 as described below. First, a ring shape gasket 50 is inserted between the rear face of the electrode section 40a and the inner circumference face of the measuring pipe 10 (an inner circumference face of a lining 30), and the axis portion 40b is inserted in a through hole 60, which is formed in the measuring pipe 10, so as to project to an outer circumference face of the measuring pipe 10. After that, a nut 80 is tightened through a base 70 onto the axis portion 40b, which projects to the outer circumference face of the measuring pipe 10.
Second Installation Structure of Signal Electrode
A second example of an installation structure of a signal electrode 40 attached to an inner circumference face of a measuring pipe 10 is shown in FIG. 9. In the second example, an electrode section 40a of a signal electrode 40 is in a shape of a funnel, and an axis portion 40b is provided so as to extend from a neck base of the funnel shape electrode section 40a. An end face 60a of a through hole 60 of the measuring pipe 10 is dented in a funnel shape so as to fit on the shape of the electrode section 40a. A ring shape gasket 50 is inserted between a rear face of the electrode section 40a and the end face 60a of the through hole 60 (a surface of a lining 30), which has been dented in the funnel shape. The axis portion 40b is inserted in the through hole 60 so as to project to an outer circumference face of the measuring pipe 10. In such a manner, the signal electrode 40 is attached to the inner circumference face of the measuring pipe 10 by tightening a nut 80 through a base 70 to the axis portion 40b which projects to the outer circumference face of the measuring pipe 10.
In the installation structure (the first and second examples) of the signal electrode, since the gasket 50 is made from elastic member such as rubber, when the nut 80 is tightened on the base 70 to the axis portion 40b of the signal electrode 40, the gasket 50 is sandwiched between the electrode section 40a and the inner circumference face (the surface of the lining 30) of the measuring pipe 10, thereby causing elastic deformation thereto, so that the gasket 50 is brought into close contact with the inner circumference face of the electrode section 40a and the inner circumference face of the measuring pipe 10, whereby the measuring pipe 10 is sealed so that the fluid flowing through the inside of the measuring pipe 10, may not leak to the outside of the measuring pipe 10 through the through hole 60. In addition, the lining 30 is formed by powder coating and has insulating capacity, but is not elastic member. For this reason, such a method of inserting the gasket 50, which has elasticity, is adopted so as to secure the seal nature thereof.
When sealing is performed by using such a gasket having elasticity, it may be important to appropriately determine a value as to a properly designed tightening tolerance in order to achieve a long life span of a product. This is because sealing cannot be sufficiently secured to the internal pressure thereof when the gasket has a small tightening tolerance, while the deterioration of the gasket is accelerated when it is tightened too much.
However, in the above mentioned installation structure of the signal electrode of the prior art, although the gasket 50 is elastically deformed by tightening the nut 80 on the axis portion 40b of the signal electrode 40, the tightening tolerance thereof cannot be handled by only controlling a torque of the nut 80 since the tightening tolerance varies widely. In order to tighten the gasket 50 with a proper design tightening tolerance, it is required that tightening tolerance of every electromagnetic flowmeter need to be adjusted.